The Humanitarian in me is urging me to give advice to help people.
The Businessman in me is telling me to shut the hell up and not give out trade secrets.
I suppose, ultimately, it depends on which voice I listen to at the time.

No problem. I can't promise that what I propose is going to be useful to you because there are a few different ways to play such ships, but those are a few things that I picked up or learned personally from playing through the line.
If you are good, you can actually forgo most of what I said and use the Baltimore and other US heavies for long range. <That is: if you are good at predicting and leading maneuvering enemy ships. I'm not the best and I've got satellite internet forcing me to not even try those long range shots against maneuverable targets lol. But it never slowed me down because there is the close-range/stealth variant that allows me to make up for it.
OH! one more thing.
Now, this is a personal choice, but remember that IFHE is NOT needed on your US Heavies. Concealment is probably the 1st tier 4 capt. skill you will want to pick up. But I would suggest going with boosting AA and secondary gun range as the second and last tier 4. CV's might not be very prevalent, but they do show up from time to time. With the US Cruisers down the whole line possessing great AA, that added range helps to cover your allies and yourself (though I will warn you now, a Des Moines can and will be killed off by AP bombers in one go by a enemy CV (has happened more than once and that alone is enough to warrant the boost in AA defense)) Also, I've found that at tier 10 (with the Des Moines) its secondary guns have actually killed a few DD's every now and then. Rather surprising really. I know it's not often, but I can think of 1 very specific occasion where my secondary's saved me and the game because they could reach out to 6 or 7km's. And have caught a few cruisers and BB's on fire from time to time. <Just a suggestion if you plan on getting really close or have no choice.
Fire Prevention is a good contender, but as a cruiser built for stealth and not intended to be shot to hell and back, I don't often let myself get caught on fire that often lol
Another very important skill to have is getting that +1 to consumables. Those radars and hydro's (as well as heals later on) have come in handy just about every game.

concealment for the US Heavy and Light cruiser lines are highly favorable considering the fact that US cruisers accuracy at range plummets due to the shells flight time. With concealment, you can get in far closer making your shots count and if things go bad, it allows you to escape alive rather than dying.
at a full concealment build, your ship's radar and concealment almost meet. This means that DD's that can spot you, can now be spotted by you. This is especially helpful because at the range of your radar, you've got the overwhelming advantage for the time that radar is active and your shells can actually hit the little buggers.
If finding yourself in a gunfight against a gunboat at a range beyond 10km's, either stop firing and go dark, or run. You'll probably never kill a Russian DD racing at 45knots or the Japanese gunboats if they are at maximum range or (hell) just outside of 10km's.
Another reason why concealment is good is that it allows you to sneak up on several of the more powerful heavy cruisers and potentially catch them off guard. Russia, France, Japan, Germany. They are all very good at long-range combat; considering that they can easily hit a Russian gunboat well enough at range. Baltimore and other US ships... not so much. They do far better at a much closer range where their shots miss less. Because at long-range, those other cruisers will certainly have far better accuracy.
Now, here's a little detail about the Baltimore and most other US Heavies. Their frontal armor is particularly good. You're not going to be tanking Battleship shells, but if you get in close, point your nose at the enemy ship and make your shells count, you will win most battles against any other cruiser in close-quarters combat. Mogami will want to turn to get it's other guns on you (switch to AP and make him regret it), Chapy's armor is far weaker than the front armor of your Baltimore (and will likely panic if caught in close combat), You had best win against the Edinburg or you need to practice a lot more, the French cruiser (whatever its name is) is a equal contender, but you should be able to outlast him or (hopefully) have allies to give you a hand. <You can attempt to force him to want to use his torps, at which time your AP will rip him apart. And the German cruiser (again, drawing a blank and too lazy to go look it up) has the same problem as Mogami with only 4 guns facing forward; making people want to turn to get the rest of the guns on target.
The US 203mm guns are lethal; both with HE and AP. The key is to know when and where to use which shell type. If something is broadside to you and within 10km's ALWAYS use AP. Further out and it can be iffy due to the plunging fire not getting through some deck armor; but is still reliable against most cruisers up to 15km's or so. Rely on HE the majority of the time until someone gives you a broadside target and is close enough to get into their Citadel.
And last thing to note; the Baltimore, like most US ships is mostly a "Support" ship. Don't try to tackle the enemy alone and be sure to support your allies. This means screen against DD's for you BB's and use your AA to defend against planes. Don't try to push up or do anything without support. In many of the other cruisers, you can afford to be a bit of a lone wolf, but the Baltimore is very reliant on being part of a team.

A secondary-based Musashi is only going to cover one situation for you. That being a DD attempting to yolo. I'm not gonna lie to you, it's very tempting to spend those skill points and modules to do so when you have a game where one of those little buggers comes running up to you and sucker punches you in the face with 10 torps. Your guns are too slow to respond to DD's and by all accounts the Musashi is incredibly weak against DD's and CV's. BUT, that is the only situation where those guns are going to play a major part in the game. So, due to limited use and highly situational, going for a secondary build is not advised. A upgraded secondary build is about as useful as the default stats; but it can... possibly (though highly unlikely) help in a fight vs a rushing DD.
Number ONE thing that you want to work on with this ship is its survivability. ALL of the fire and torpedo protection you can throw on this thing. You are always going to Eat a metric crap-ton of HE and you are almost certainly going to eat torps if things go south... or north... or east or west... I mean, torps are iffy if you know how to read a map... but you will still be getting hit by them on a regular basis if you do any sort of pushing.
The second thing to improve (personally for me) is the concealment. Works for when you are sniping and works great when you are pushing up with the cruisers. When sniping, you disappear from view for 10 seconds after 20 seconds as your guns spend 30 seconds to reload. Not to mention the added benefit of seeing a cruiser or other BB crap themselves when a Musashi suddenly appears roughly 13km's away from them. For a ship that can snipe you halfway to Mars, it's kinda intimidating to have the thing suddenly showing up on your front door.
Another reason for concealment is that the Musashi is a good sniper, BUT! the moment your cruisers are dead (and you best face the fact that the cruisers HAVE to get closer to the enemy than you do in order to get into range of their guns) YOU are dead. The Musashi CAN NOT take on a DD alone. The only DD's that it can possibly take on are those that make themselves visible (and at tier 9, good luck with that). Without cruiser or DD support, that enemy DD can do circles around you and you won't have a chance in hell of ever seeing it. This goes for planes as well. You might be surprised to find that you can take out maybe 2 planes in a squadron, but that's it... total. The CV will wreck you and nothing is easier to hit with planes than this behemoth.
So, where do you position yourself? For me, I roll right up to the enemy as long as I know I'm not running into a pack of DD's. This I find out by simply holding a bit behind the cruisers until DD's start capping, or other tells. If I feel reasonably assured I'm not going to have a wall of torps coming my way, I tank for the cruisers that I HOPE (though some players are scared littles %*&^S) are screening me from DD's and not running away. The Musashi has a weak point in the front but as long as you make the rounds count, your guns and ship will win in a 1 on 1 vs nearly any other BB and you will massacre any and all cruisers in your path. Hint: Don't "Angle". Point your nose directly at the enemy ship.
If I die before my cruisers, no problem. I'll have certainly killed something and/or shot up something enough to total my own HP (assuming I don't get swarmed). Not losing any money either so that's a bonus. But if my cruisers die without taking out enemy DD's or the CV, I'm nothing but a XP piñata for them. So, I much prefer supporting my cruisers on the front lines rather than sniping in the back. Now, i do plenty of sniping, but only when I'm traveling to my next waypoint or if some poor sap is obliviously showing me their broadside.
But, if you want to snipe from the back, the Musashi can do that too very reliably as it can easily hit most anything if you know how to properly lead a target. The only BB's that you should fear getting up close to are tier 10's but... I've honestly only met a handful of tier 10 players that know how to properly brawl. Usually, they see me coming and turn to run.
A Yam, and equally, another Musashi are your only real threats as far as BB's go if you know how to handle this ship, Cruisers are lunch if they aren't hiding behind a rock, and DD's and CV's are the bane of your existence.
My preference; and in this order: Total 12 Capt. Points.
1-Preventative maintenance (because you have to have at least one level 1 skill)
2-Repair reload boost (optional but preferred) it helps but can be passed if you know how to time your repairs accordingly,
3-!!!Fire and Flood time reduction (both the skill and module)!!! don't ask questions you'll want these,
4-!!!fire prevention!!! don't think about it get it, Concealment (optional but preferred) always useful but not a requirement,
5-Adrenaline rush (optional but preferred) You have plenty of HP to keep you in a fight even when I the red. You'll get much use out of this skill,
Optional:
+1 to consumables (if you want that extra heal),
Concealment (if you want to hinder return fire from other sniping BB's and cruisers, and/or close the distance)
+1 to deployable spotter/fighter plane as they help to detect torps, interfere with CV bomb/torp drops, and on a very rare occasion can detect a DD that strays too close.
Modules:
Honestly... anything that increases your survivability and reduces the time it takes those guns to fire and turn.

1: Chapy. The ship I go to in order to complete "set on fire" missions and "hit targets with main guns" missions. Doesn't matter the tier.
2: Mogami. Same as above but with more protection. Same as the Chapy with being up-tiered. You (potentially) won't live as long, but the amount of damage you can put out still makes you lethal to anything.
3: Cleveland. Still a gud ship. For tier 8, it's powerful. Tier 10, it at least can light some fires from behind a rock.
4 (or 7th): Edinberg. Extremely dependent on the situation. Either a good ship or completely useless/dead at higher tiers.
5: Baltimore. Not a bad ship for what it's good at. Suffers at higher tiers though.
6: Hindenburg. Chapy does ranged combat better.
7: Charles. Hindenburg does it better still.
Top 3 are my personal best for what they do.
Chapy is far better at HE spam than any of the other ships due in part to range, time to target, and number of guns. I find it better at ranged combat than the Hindenburg even though it is far less tanky. The addition of radar makes it far more viable to engage (and win) against DD's too. Not to mention, if you want access to a spotter plane, just don't upgrade the hull. Reason why it's my preferred choice is because it can up-tier well. It may eat citadels if/when a BB looks its way, but so will most any other tier 8 cruiser. (Good against BB's, Cruisers, DD's, and a few Planes) Russian Bias all the way.
Mogami is my second favorite considering the customization. Though I commonly stick with the 155's more often than not. It can dish out damage fast and can rip a DD apart in seconds. If able to fire from a safe position or just running out in the open, it can burn most anything to the sea floor while tanking a few hits. (Good against BB's and cruisers / Needs help to take on DD's (spotting them) and it might as well not have AA.)
Cleveland can do much the same as the other two. Difference being its lack of armor at higher tiers and overall ability to hit some targets at max range. It does however excel at close to mid-range combat against certain targets. With the amount of DPS, you can at least hurt the enemy quickly before you die and you don't have to worry about DD's perma-spotting you like in a Mogami, Hindenburg, or Charles. (Good against BB's (when hiding), DD's, planes, and situationally cruisers.)
Edinburg isn't bad, but it is a gamble every time you take it out. If you get stuck in a tier 10 match, some of the DD's in higher tiers can rip you apart; let alone high tier cruisers. It's great at tier 8. Sucks to be in it at tier 10. (dead in seconds, or you manage to get close enough to actually hit something... but then it also depends on if the thing you can hit is showing its sides or weakpoints.)
Baltimore is capable at nearly anything. Not suffering from much but not particularly special. Slow rate of fire with its 203's makes it heavily reliant on support for nearly anything it attempts to kill however... unless you can catch a enemy showing its broadside. Then the Baltimore will tear it apart in a heat beat. Placed here because it has radar and can deal with more threats than the lower two. (Weak to BB's and cruisers when out in the open / good against planes and capable against DD's and cruisers at close range)
Hindenburg is tough as nails but it's guns are few. Kiting is ok, but the Chapy does it way better. Only thing it has over the Chapy is survivability and that only works at same-tier games. Higher-tier games and you get cit'd on like all the rest. (takes on cruisers well, highly situational use of torps allows it to possibly kill a BB / Weak to DD's and planes)
Charles is fun. Not the worst, but not the most original. It can Kite but Hindenburg does it better. It can tank, but Hindenburg does it better still. It's got a few quirks but nothing in particular that stands out or that beats any of the others... if you're not taking consumables into consideration. (Good against other cruisers... gonna need help taking on much of anything else though.)

when the game crashes, shut the game down, Ctrl+alt+Del, do whatever you have to in order to close the program...
AND THEN
Start the game back up.
It's that simple.
As long as you reconnect within 5 minutes you will be perfectly fine and not ruin other players experiences. Heck I've seen people start up in 10 minutes and not get flagged. Did it myself.
If it takes you longer than 5 minutes to start the game back up and reconnect, then you might want to get a better machine or internet provider.
If your crashes are anything like mine, you shouldn't have a problem getting back into the game after the initial error. If for some reason the thing just refuses to start back up at all, then you might want to go ahead and try to find out why that is before attempting to play.

As someone who absolutely loves his Musashi and knows about said "weakness", I do not agree with you on that.
These ships are damn near OP when it comes to fighting other BB's and Cruisers. Even Vs. one another, the skill to accurately hit that single vital point is a must and has to be exploited otherwise a one on one against a angled Yam or Musashi would result in most any other BB getting the snot beat out of it.
A bow-on Yam/Mush are beyond lethal in CQB vs any other BB and outright overpowered vs a cruiser. The other ships only saving grace is to hit that weak point IF/when presented. As someone who owns the Musashi, I know for a fact that that weakpoint is only guaranteed if you allow the enemy in front of you or to your side to get a proper shot on it. Pointing your nose directly at your enemy actually greatly reduces the chances of him attaining the angle needed to hit your citadel.
The Musashi (and I would suspect the Yamato) are tough as nails and hard as hell to kill even when taking citadel hits. But while the enemy has to hit that exact spot, or your fully exposed broadside, you merely have to aim at the general location of where your enemies citadel is located (even through the bow of some ships) and you can get to it. Those 460mm guns go through bow armor like it was tissue paper and I don't have a problem with other ships being able to expose about the only weakness that either Yam or Musashi should/would be exposing.
They still have to accurately hit that location either way. At long range, their chances of hitting it are slim (if you aren't sitting still or maneuvering like you should). At close range, your guns will beat theirs and you should have support nearby regardless making any such attempt a suicide run for any ship trying to get that close to you. So, it's not easy at long range to hit it and at short range you should be pointing directly at your biggest threat and/or blowing it out of the water in a couple of volleys.

WW1 was probably the last time forts had any major impact on any battle strategy involving warships. Even then, the exchange between the ships of that time and the forts arsenal were about even. Only big thing being, the fort didn't have to worry about sinking. Forts at that point had to be taken into consideration and any assault by ship had to be thoroughly planned out and gain/losses weighed.
During WW2, warships were sporting more accurate and powerful artillery that could be directed to a target outside line of sight. Stationary Forts were also typically avoidable and only those in the most strategic locations would have to have been bothered with at all. Notice how few were actually dealt with though by warships. Most forts that played any significance during WW2 were mainland and dealt with ground forces, but even they only lasted as long as it took the attacking side to bring up long-range artillery.
Now, that's not to say that sea forts didn't go up during WW2. There are the https://en.wikipedia.org/wiki/Maunsell_Forts which were used to "defend" the UK. But I seriously doubt that they'd have stood much of a chance against a enemy warship of cruiser class... maybe could have done ok against a Destroyer. Chances are, they might could probably/potentially deal some damage, but I'd personally say that they'd have been wiped out had something gone after them. Now, as a AA platform, I'm sure they would have/did do wonders... if the enemy was within range.
The last major "fort"ification that Warships had to deal with directly were the Japanese controlled islands with their massive tunnel networks and artillery imbedded in the sides of mountains. Those guns didn't really do much at all to the warships and only really came into play when the transport ships neared the islands. The Battleships were practically uncontested as they lobbed shells into the island. You can imagine just how bad it would be if the Japanese had constructed a concrete "target" and stuffed all of their weapons into it instead of spaced out over an entire island.

I've used my Xbox 360 controller to play the game on my computer.
Thing is, you have to use another program in order to do so. "Controller Companion" on the steam workshop allows you to use it (with a bit of fiddling around).
It was something to do and it did work, but I wouldn't suggest it because it's not as intuitive. You can setup the buttons as you see fit through the program. I personally didn't put that much effort into setting up the button layout.
But I will say, it was fun to mix it with another program. "Voice Bot" another program that allows you to speak words and tie those words to keyboard keys really does free up a lot of your time. Only problem with that thing is that you've got to set it up and work with it for it to recognize your voice.
Aiming and moving can be done with joysticks and whatnot, Weapon systems can be setup with the buttons, aiming with the tabs, firing with triggers, got to have your Cntr and Alt keys there somewhere, and for consumables I simply just used voice bot. Of course, you might not want to use it if you share a house or apartment with someone... people might think you are talking to youself.

My favorite would have to be the French Line out of them all.
Though I completed the US line first, it had it's moments of aggravation and trail and error; same too for the other lines.
But the French line is just one steady improvement after another. There is no real learning curve or changing tactics to deal with. Each ship performs the same role as the one before it and with that, there are less times where I had to figure out what I was doing wrong. Even better, each tier up is a pure upgrade with no potential detriments like some other nations cruiser lines have.
As far as individual ships, nations, and other things go; I've got my preferences and every line is/was enjoyable. But, as for "Which cruiser line was my favorite to grind through" I've got to give it to the French as I've enjoyed every ship from tier 1 to (at least for now) tier 8 with a little less hassle than the others. Not winning any awards for creativity among them, but there were far less "annoying" moments.
Russian line is a close second of course; probably even tied with the French. Powerful, long-range ships but they seem to make far easier targets (for whatever reason) with regards to retaliatory shots. French line seems to let me get out of a sticky situation a bit easier than the Russian line which, more often than not, ends up getting the snot kicked out of it if I make a mistake.
The only thing that threw me for a loop in the German line was the York. Fun ship (and one of my favorites at that tier), but man did that take some time to figure out XD
US line had a couple hit and miss and rather annoying in the mid-tiers, but ends strong with one of the best cruisers in the game as far as I'm concerned.
British line starting off is/was just painful. Those instant KO's aren't any fun at all (if you are on the receiving end).
Japanese line is nice. Not bad, but a little slow as far as early tiers are concerned. Tiers 6,7, and 8 (stopped there for now) are fun.

Only way I keep a ship is if A: I have a permanent camo for it, or B: I enjoy playing it.
The camo'd ones are worth keeping because, with the camo, you would have to actually work to lose credits and the extra xp helps with grinding missions and events. Also good for quickly leveling new captains if/when you need some new ones trained.
As for enjoying playing them, well... that's obvious. But if you need the credits (and you'd prefer not to grind for them in your ship), you can always sell the ship and later repurchase it if/when you want. I'll tell you now though, it will cost you a bit to repurchase them and upgrade them again, best to leave the upgrades on it if you plan on picking it back up. But you will have to fully repurchase the modules (or use gold). <which might not be worth it to you.

Did a quick search through wiki. As far a "National Flavor"... I can't really think of much. There's some basic stuff you can throw on them I guess. Spotting range, speed variables for both above and below water, diving times, detection ranges, etc.
US: https://en.wikipedia.org/wiki/Gato-class_submarine <huge list of design "gimmicks" to choose from I suppose. Depending on type, US had larger subs as far as the Gato is concerned. From its description, It submerged slower than the smaller subs and turned slower too; due to it being so large in comparison. As far as this specific design is concerned, I'd say that it would be able to remain submerged longer than others and/or have increased HP... though I don't know just exactly how useful that extra HP would be... I suppose it would help (or be a detriment) if/when you start taking hits from depth charges.
German: Better periscope (submerged) visibility range? Seems a good, fitting thing for this nation. BUT, Germany was the king at one point in time with WW2 submarines... here's a potential list of things: Better underwater performance in general. So, counter to the French, faster underwater speeds with better turning?
With the increasing sophistication of Allied detection and subsequent losses, German designers began to fully realise the potential for a truly submerged boat. The Type XXI "Elektroboot" was designed to favor submerged performance, both for combat effectiveness and survival. It was the first true submersible. The Type XXI featured an evolutionary design that combined several different strands of the U-Boat development program, most notably from the Walter U-boats, the Type XVII, which featured an unsuccessful yet revolutionary hydrogen peroxide air-independent propellant system. These boats featured a streamlined hull design, which formed the basis of the later USS Nautilus nuclear submarine, and was adapted for use with more conventional propulsion systems. The larger hull design allowed for a greatly increased battery capacity, which enabled the XXI to cruise submerged for longer periods and reach unprecedented submerged speeds for the time.
Japanese: More lethal torpedoes. Obviously those things are going to be powerful if you compare them to the Japanese DD's torps. Not sure exactly what other thing you could do to them to make them "unique". Obviously guided torpedoes is out of the question.
French: I was thinking a speed boost usable above surface only... not really original, but it is apparently a "French" thing. But, unlike a lot of the other wiki pages on specific nations submarine designs, this one actually does generalize them. So... potentially: Good rudder response above water and/or better torpedo firing arcs. At the expense of slightly less efficient diving speed?
While being designed by different bureaux, the French submarines of this period shared a number of features. They were generally double-hulled, with an emphasis on good surface handling, though this led to them being indifferent divers. They also emphasized habitability, needing to be suitable for service in France's colonial empire, which could mean long voyages and operations in the tropics.
One unique feature was the use of external torpedo mounts. As well as torpedo tubes in the bow and stern, most French submarines carried torpedo tubes externally in trainable mounts, built into the outer casing. These could be trained to fire at various angles beyond fore and aft, but could not be re-loaded at sea. French submarines also mounted torpedoes of different calibres, typically carrying the 400mm (15.75 inch) torpedo, for use against "soft" targets such as merchant ships, as well as the 550mm (21.7 in) torpedo for use against warship.
Also, with a few unique designs, the French actually had a few "submarines" that were really just submersible ships. There is one design I saw that actually had two dual gun turrets... though now I can't recall 100% if it was a French submarine.
British: Not too sure about this one... I guess, they could have one of the longest dive times before needing to surface?... of course this would be at the expense of several other factors.
The second group of the U class consisted of twelve submarines, of a similar design to the original three, but most were modified to improve depth keeping. <whatever that means.
.

Isn't that how the game works out the vast majority of the time. More often than not, the enemy will come to you. You've got a equal chance of the enemy team going full lemming to the other side of the map, but eventually, they will make their way there to you. WG can fiddle with the actual top speed a little for gameplay sake if they need to, so there is that too.
But even at a lower max speed compared to other ships, this is also workable simply by reading the map and putting yourself in the path of the enemy. Now, at 20knots, you aren't chasing anyone down, but you can darn sure get into position to intercept a approaching enemy. Which is why (counter to the placement of CV's) subs should be started ahead of the fleet. 20knots to move to a cap or wherever and at that 20 knots, eventually, you can intercept the path of a enemy ship that is moving/hiding in the rear. They can't outrun you because there is a border wall keeping them in.
As far as the game is concerned with this issue, the literal "framing" of the game takes care of these "circumstances".
I'm sure there are plenty of ways to work out the details. Anti-sub warfare is something that will be seen in this upcoming Halloween mission. Depending on how they go about balancing things, submarines might not be as easy to deal with as one might think, or they might very well have other tricks/tactics up their sleeves to further play around with. It's hard to tell exactly how weak or OP submarines will be without any clue what to expect to begin with. <Which is why WG stated that they'd see how they do in the upcoming scenario whether or not they'd consider it further.
I really hope that people aren't expected submarines to be the defacto factor of a victory every time. Sure, they would be good for spotting up until they have to dive for safety, good at sniping key targets (if/when they can manage), and good at distracting enemy forces (capping, attacking, or otherwise), but they are not going to be any more effective than any other ship class. But, at the same time, they are also NOT going to be any less effective than any other ship class either. They do share similarities with DD's, that's a given, but their specific features and traits as well as tactics are different enough to potentially make them stand out.
But the way I see it they, like the DD's (with their stealth) and BB's (with their huge offensive capabilities), certainly can and will play a significant role in some victories. It would be a different form of play style, a slower-paced style of gameplay (with moments of action here or there) that rewards those who know how to put themselves in front of a rushing enemy force and keep their cool. DD's finding themselves dealing with a overwhelming force are running scared or avoiding it; not so in a submarine. Keep your cool and hope for the best. Plan your shots and have a escape plan ready.
Heck, it might even be a significant counter to some enemy forces that just bum-rush one side of the map and continue to steamroll to the other.
A single sub, waiting in the shadows as the enemy fleet goes sailing by, hoping that the enemy doesn't manage to spot him. Waiting till the coast is clear before firing off a few torps into a BB or a enemy CV that is following the main group. Instantly, that rushing (overwhelming) enemy fleet now has to read the situation and slow their roll. There's a easy to kill enemy nearby, but the only way you are going to spot it is if it either surfaces for air, or your DD's and cruisers hunt it down. That Yamato had no problem pushing up, it can tank, and the ships in front of it could easily screen it from a DD. But that one submarine managed to slip past them.
For context, I love the Musashi and the last thing I want is another ship class that can counter it, but if there's no DD's or CV's on the enemy side (and, in certain situation, even if there are), I have no worries yoloing up to a cap or area on the map and laying waste to everyone I can see. I don't expect subs to change much, but they could give me something to be wary about and if he/she manages to slip past my teams defenses and takes me out, all I can say is that the kill was well deserved. In addition, as a DD, it would give me a target to go hunt that my class is exceptional at dealing with. Now, instead of just hunting down straggling BB's, I've got a option to actively defend my fleet against a foe few others can deal with.
Either way, submarines played a massive part in many iconic ships destruction and was a huge factor in naval warfare at the time. So much so that many ships were outfitted with the necessary equipment to deal with them; as any other vessel or threat.
Don't know if WG wants to deal with everything involved to introduce them to regular gameplay, but it might be worth it to introduce this last piece of the naval warfare experience. It's certainly not going to be hard to balance that's for sure. These things come with their own peculiar balancing with their lack of armor and requirement to surface from time to time. Just tweaking the surface time requirements alone would be enough to deal with how effective these things are.
Also, just to add, I'm not sure if Submarines are worth counting as DD's or if they would be better off being limited as we see currently with CV's. There are a good number of cruisers that should be equipped to deal with Submarines about as well as DD's, so I'm thinking that submarines might could take up the same slot as a DD. So long as there aren't one-sided battles where one team ends up with something like 5 submarines vs another team with 5 DD's. DD's being considered more than capable of countering those submarines... but then again, we do see on the reports just how many DD's those submarines took out. So I'm not 100% sure exactly which one is better at countering the other in terms of what we'd see in the game.

So, direct kills?
Well, we all know the Indianapolis. And you can bet that the majority of DD kills was from the submarine. About 30 DD's were said to have been taken out by submarines and if they can 1 shot a cruiser, I'm inclined to think that if a single torp hits a DD, it's game over; except of course in a few instances.
https://en.wikipedia.org/wiki/List_of_submarine_actions
1939, October 14 – German U-boat U-47 sinks HMS Royal Oak in Scapa Flow base. The First Lord of Admiralty Winston Churchill officially announced the loss of Royal Oak to the House of Commons, first conceding that the raid had been "a remarkable exploit of professional skill and daring". <I'd say this would make for a good scenario mission.
1941, November 13 – U-81 strikes HMS Ark Royal with a single torpedo. She sinks the following day due to crippling damage. <flooding damage is no joke.
1942, September 15 – Japanese submarine I-19 sinks U.S. aircraft carrier USS Wasp and destroyer USS O'Brien and damages battleship USS North Carolina with a single salvo.
1944, November 29 – USS Archer-Fish sinks Japanese aircraft carrier Shinano, the largest vessel of that time. <took 4 torp hits to do it, but it worked.
This is just the wiki list of ships sunk by submarines. There are certainly a lot more because I know that the DD's aren't listed.
Submarines didn't need outside help to kill some of the largest ships during the war, just the opportunity to do so. With the games border restrictions and contested locations, this makes finding those enemy ships that much more simple.